Extreme ultraviolet (“EUV”) light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates such as silicon wafers. Here and elsewhere, it will be understood that the term “light” is used to encompass electromagnetic radiation regardless of whether it is within the visible part of the spectrum.
Methods for generating EUV light include converting a source material from a liquid state into a plasma state. The source material preferably includes at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV part of the spectrum. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by using a laser beam to irradiate a source material having the required line-emitting element.
One LPP technique involves generating a stream of source material droplets and irradiating at least some of the droplets with laser light. In more theoretical terms, LPP light sources generate EUV radiation by depositing laser energy into a source material having at least one EUV emitting element, such as xenon (Xe), tin (Sn), or lithium (Li), creating a highly ionized plasma with electron temperatures of several 10's of eV.
The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma in all directions. In one common arrangement, a near-normal-incidence mirror (often termed a “collector mirror” or simply a “collector”) is positioned to collect, direct (and in some arrangements, focus) the light to an intermediate location. The collected light may then be relayed from the intermediate location to a set of scanner optics and ultimately to a wafer.
In some LPP systems each droplet is sequentially illuminated by multiple light pulses. In some cases, each droplet may be exposed to a so-called “pre-pulse” and then to a so-called “main pulse.” It is to be appreciated, however, that the use of a pre-pulse is optional, that more than one pre-pulse may be used, that more than one main pulse may be used, and that the functions of the pre-pulse and main pulse may overlap to some extent.
Typically, a pre-pulse may cause some or all of the source material to heat, expand, gasify, vaporize, ionize, generate a weak plasma, or generate a strong plasma, or some combination of these, and a main pulse may convert most or all of the material affected by the pre-pulse into plasma and thereby produce an EUV light emission. Pre-pulsing may increase the efficiency of the source material/pulse interaction due to a larger cross-section of material that is exposed to the main pulse, a greater penetration of the main pulse into the material due to the material's decreased density, or both. Another potential benefit of pre-pulsing is that it may expand the target to the size of the focused main pulse, allowing all of the main pulse to participate in conversion of the source material into a plasma. This may be especially beneficial if relatively small droplets are used as targets and the irradiating light cannot be focused to the size of the small droplet. Thus, in some applications, it may be desirable to use pre-pulsing to increase conversion efficiency and/or allow use of relatively small, e.g., so-called, mass limited targets. The use of relatively small targets, in turn, may be used to lower debris generation and/or reduce source material consumption.
The main pulse and the pre-pulse, if it is used, must generally be directed and focused onto the droplets of source material by using a focusing system. Generally in a high power focusing system for an EUV LPP source the optical elements in the beam path absorb some of the energy from the main pulse and the pre-pulse causing heating in those optical elements. This heating creates a thermally-induced optical distortion (also known as a “thermal lens”) by virtue of the temperature dependence of the refractive index and the coefficient of thermal expansion of the material making up the optical element. The thermal lens alters the optical power of the optical elements causing deviations from optimal values.
It would thus be advantageous to be able to negate the effects of thermal lensing and so to avoid the performance limitations it imposes.